Ion implantation has been utilized to treat the surfaces of a variety of materials, including metals, ceramics and plastics, to change the surface characteristics of the materials. In a conventional ion implantation process, ions are formed into a beam in an ion gun and accelerated to high energy before being directed into the surface of a solid target. Such conventional ion implantation is utilized not only for surface treatment of materials to enhance hardness, wearability and so forth, but is also utilized in the microelectronics industry to implant ions into semiconductors to provide selective doping of the semiconductor. In the latter application, it is essential that the ions implanted into the semiconductor be carefully selected so that substantially only the desired ions are implanted.
The high cost of conventional ion implantation techniques limits the practicality of such techniques for surface treatment applications, e.g., surface hardening. Recently, a more efficient technique for implantation of ions into essentially all surfaces of a target object simultaneously from a plasma around the object has been developed. Such a technique, commonly called plasma source ion implantation (PSII) is disclosed in U.S. Pat. No. 4,764,394 by Conrad, issued Aug. 16, 1988. The plasma around the target is the source of the ions which are implanted in the target. A high negative potential pulse is applied to the target to accelerate ions from the plasma across a plasma sheath toward the target in directions substantially normal to the surfaces of the target at the points where the ions impinge upon the surface of the target. Multiple pulses are typically applied to the target in rapid succession to perform multiple implantations until a desired concentration of implanted ions within the target is achieved.
In the PSII process, the creation of the plasma may be carried out in a variety of ways, typically by ionizing a neutral gas within the evacuated chamber surrounding the target. The ions may also be formed in an area separate from the main chamber and then transported into the main chamber where they form a plasma which is confined around the target. It is commonly the case that unwanted species of gas molecules are mixed with the neutral gas from which the ions are to be obtained. For example, nitrogen is often used for nitride ion implantation to improve the surface hardness of metal tools, but other gases may be found within the evacuated chamber with the nitrogen, e.g., oxygen mixed with the nitrogen, or even heavy molecules such as organic molecules that may be found within the evacuated chamber from outgassing of materials in the chamber or from leaks. These unwanted species may also be ionized and may be implanted from the plasma into the target. Generally, the presence of the unwanted ion species may be acceptable where the plasma source ion implantation is for surface hardening of many materials, but can be particularly detrimental where the electrical, mechanical, or chemical properties of the surface are crucial, as in semiconductor processing.
Furthermore, dopants used in semiconductor processing are often ionized from a working gas containing other elements, for example, boron trifluoride in the case of boron as the dopant, which inevitably generates ions of unwanted species. In conventional ion beam ion implantation, the unwanted ions are removed from the beam before it impinges on the target. However, in conventional plasma source ion implantation, all of the ionized species found in the plasma may be implanted.